The present invention relates to deriving a time reference from time of arrival measurements to enable vibration events to be identified and analysed. It finds particular, but not exclusive, utility in deriving a time reference from time of arrival measurements derived from a gas turbine engine rotor, such as from blade tips of such a rotor.
Blade tip displacement data, derived from time of arrival measurements, is analysed to identify vibration events and trends. In order to correctly identify whether vibration is synchronous, that is it occurs at a multiple of the angular speed of rotation of the rotor to which the blades are mounted, or asynchronous, that is it occurs at a non-integer multiple of the angular speed of rotation, it is necessary to have an accurate time reference. Conventionally this time reference is provided once per revolution of the rotor and is known as a once per revolution (OPR) signal. The OPR signal is conventionally provided by a special feature on the rotor which produces a time of arrival measurement that differs from that produced by other features, for example having a different magnitude. Vibrations and other events and trends noted in the time of arrival measurement data can then be compared to each other by their position relative to the OPR signal.